1. Field of the Invention
The present invention relates to a swash-plate, plunger-type hydraulic pressure apparatus such as a swash-plate, plunger-type hydraulic pump or motor, and more particularly to an attachment structure for a retainer plate which prevents shoes from being lifted off a swash plate during operation of such a swash-plate, plunger-type hydraulic pressure apparatus.
2. Description of the Prior Art
One conventional swash-plate, plunger-type hydraulic pressure apparatus is shown in FIG. 14 of the accompanying drawings. As shown in FIG. 14, the prior swash-plate, plunger-type hydraulic pressure apparatus includes a cylinder block 1 having an annular array of cylinder bores 1a surrounding an axis C1. A plurality of plungers 7 are slidably fitted in the respective cylinder bores 1a. The plungers 7 have spherical ends 7a to which respective shoes 2 are swingably coupled. The shoes 2 are slidably held against a sliding surface 8a of a swash plate 8. When the cylinder block 1 rotates about the axis C1, the shoes 2 slide on the swash plate 8 and the plungers 7 reciprocally move in the respective cylinder bores 1a. In order to prevent the shoes 2 from being lifted off the sliding surface 8a, the shoes 2 are held against the sliding surface 8a by a retainer plate 3. The retainer plate 3 is held in position by a holder plate 5 that is fastened to the swash plate 8 by bolts 6.
The swash-plate, plunger-type hydraulic pressure apparatus of the above structure is disclosed in Japanese Laid-Open Patent Publication No. 2-102958, for example.
The reasons why the shoes 2 are held against the sliding surface 8a by the retainer plate 3 will be described below in specific details.
The plungers which operate in a pumping action switch from a discharge stroke to a suction stroke at their top dead center in the cylinder bores. Since the direction of movement of the plungers in the discharge stroke is opposite to the direction of movement of the plungers in the suction stroke, it is necessary that forces strong enough to overcome the inertia of the plungers in the discharge stroke be applied to the plungers at their top dead center so as to start the suction stroke.
During the pumping action, the plungers draw oil because of their own movement in the suction stroke. It is necessary to impose forces on the plungers to enable the plungers to make their own movement to draw oil.
In the case where the swash-plate, plunger-type hydraulic pressure apparatus is of the variable displacement type, the inclination of the swash plate is varied to change the displacement. When the inclination of the swash plate is varied, the shoes would be spaced from the swash plate unless the shoes were moved in a manner to catch up the variation in the inclination of the swash plate. For example, if the angle of inclination of the swash plate is varied while the plungers are in a lower-pressure stroke, the shoes would be lifted off the swash plate. When the plungers are then shifted into a higher-pressure stroke, the plungers would be pushed by the high pressure in the cylinder bores, forcing the shoes to hit the swash plate, producing noise and reducing durability.
As shown in FIG. 15 of the accompanying drawings, if the center G1 of gravity of each shoe 2 is spaced from the center O1 about which the shoe 2 is swingable with respect to the plunger 7, then the centrifugal force F1 which is applied to the shoe 2 as the cylinder block 1 rotates acts as a force tending to turn the shoe 2 about the center O1. Therefore, the shoe 2 is liable to be tilted with respect to the sliding surface 8a, and its lower surface tends to be spaced from the sliding surface 8a. The lower surface of the shoe 2 has a groove 2a defined therein which serves as a hydrostatic pressure bearing. The groove 2a is supplied with hydraulic pressure from the cylinder bore 1a through an oil passage (not shown) defined through the plunger 7 and an oil passage defined through the shoe 2. If the shoe 2 were lifted off the sliding surface 8a, as described above, the oil would leak out of the groove 2a, and the performance of the static pressure bearing would be lowered.
The retainer plate 3 is employed to meet the above requirements and to avoid the above drawbacks. To hold the shoes 2 against the sliding surface 8a with the retainer plate 3, the retainer plate 3 is required to be positioned in place. In FIG. 14, the retainer plate 3 is held by the holder plate 5 through a bearing 4 interposed therebetween. The retainer plate 3 may instead be held directly by the holder plate 5 without any bearing therebetween. Alternatively, an inner circumferential side of the retainer plate 3 may be pressed by a spherical ring or the like.
With the shoes 2 held against the sliding surface 8a by the retainer plate 3, as described above, the retainer plate 3 is kept in contact with the other member, such as the bearing, the holder plate, or the spherical ring, than the shoes 2. The retainer plate 3 is therefore subject to a rolling resistance or sliding resistance imposed by one of these other members. In addition, the number of components used is increased by the other member that is required to position the retainer plate 3.
During the pumping action of the swash-plate, plunger-type hydraulic pressure apparatus, only those shoes which are associated with plungers that operate in the suction stroke are actually required to be held against the swash plate. Those plungers that operate in the discharge stroke are pressed against the swash plate under forces produced in reaction to the discharge pressure of the plunger. If the retainer plate is attached in engagement with all the shoes, then the retainer plate can be positioned under pressing forces that act on those plungers which are in the discharge stroke. As a result, any one of the above other members for positioning the retainer plate may be dispensed with.
Japanese Laid-Open Patent Publication No. 2-102958 discloses an apparatus having an annular disk plate disposed between a swash plate and shoes, and a retainer plate positioned upwardly of the shoes, the disk plate and the retainer plate being fastened to each other by bolts with the shoes clamped between the disk plate and the retainer plate. With this arrangement, the pressing forces acting on those plungers that are in the discharge stroke are applied to hold the bolted plates on the swash plate, thus positioning the shoes that are clamped between the disk plate and the retainer plate. Consequently, no special member is required for positioning the retainer plate, and the above problem of rolling or sliding resistance is eliminated.
The disclosed apparatus is, however, disadvantageous in that the disk plate is of a special shape, cannot easily be manufactured, and is highly expensive to manufacture. Therefore, the cost of the apparatus is relatively high, the number of components of the apparatus is large and so is the weight of the apparatus.
FIG. 18(A) of the accompanying drawings shows a conventional shoe composed of members 9a, 9b that are threadedly fastened to each other with a retainer plate 3 clamped between the members 9a, 9b. The shoe is thus held on the retainer plate 3.
FIG. 16(B) of the accompanying drawings illustrates a conventional shoe comprising members 9c, 9d that are welded to each other with a retainer plate 3 clamped between the members 9a, 9b. The shoe is also held on the retainer plate 3.
In the structures shown in FIGS. 16(A) and 16(B), however, the members 9a, 9b and 9c, 9d are required to have a relatively large thickness where they are threaded or welded so that their joined regions have a high mechanical strength. As a consequence, the shoes are necessarily large in size, and the entire apparatus is also large in size and made up of a large number of components.